The present invention relates to precision sand casting of engine cylinder blocks, such as engine cylinder V-blocks, with cast-in-place cylinder bore liners.
In the manufacture of cast iron engine V-blocks, a so-called integral barrel crankcase core has been used and consists of a plurality of barrels formed integrally on a crankcase region of the core. The barrels form the cylinder bores in the cast iron engine block without the need for bore liners.
In the precision sand casting process of an aluminum internal combustion engine cylinder V-block, an expendable mold package is assembled from a plurality of resin-bonded sand cores (also known as mold segments) that define the internal and external surfaces of the engine V-block. Each of the sand cores is formed by blowing resin-coated foundry sand into a core box and curing it therein.
Traditionally, in past manufacture of an aluminum engine V-block with cast-in-place bore liners, the mold assembly method for the precision sand process involves positioning a base core on a suitable surface and building up or stacking separate crankcase cores, side cores, barrel cores with liners thereon, water jacket cores, front and rear end cores, a cover (top) core, and other cores on top of the base core or on one another. The other cores can include an oil gallery core, side cores and a valley core. Additional cores may be present as well depending on the engine design.
During assembly or handling, the individual cores may rub against one another at the joints therebetween and result in loss of a small amount of sand abraded off the mating joint surfaces. Abrasion and loss of sand in this manner is disadvantageous and undesirable in that the loose sand may fall onto the base core, or may become trapped in small spaces within the mold package, contaminating the casting.
Additionally, when fully assembled, the typical engine V-block mold package will have a plurality of parting lines (joint lines) between mold segments, visible on the exterior surface of the assembled mold package. The external parting lines typically extend in myriad different directions on the mold package surface. A mold designed to have parting lines extending in myriad directions is disadvantageous in that if contiguous mold segments do not mate precisely with each other, as is often observed, molten metal can flow out of the mold cavity via the gaps at the parting lines. Molten metal loss is more prone to occur where three or more parting lines converge.
The removal of thermal energy from the metal in the mold package is an important consideration in the foundry process. Rapid solidification and cooling of the casting promotes a fine grain structure in the metal leading to desirable material properties such as high tensile and fatigue strength, and good machinability. For those engine designs with highly stressed bulkhead features, the use of a thermal chill may be necessary. The thermal chill is much more thermally conductive than foundry sand. It readily conducts heat from those casting features it contacts. The chill typically consists of one or more steel or cast iron bodies assembled in the mold in a manner to shape some portion of the bulkhead features of the casting. The chills may be placed into the base core tooling and a core formed about them, or they may be assembled into the base core or between the crankcase cores during mold assembly.
It is difficult to remove chills of this type from the mold package after the casting is solidified, and prior to heat treatment, because the risers are encased by the sand of the mold package, and may also be entrapped between the casting and some feature of the runner or risering system. If the chills are allowed to remain with the casting during heat treatment, they can impair the heat treatment process. The use of slightly warm chills at the time of mold filling is a common foundry practice. This is done to avoid possible condensation of moisture or core resin solvents onto the chills, which can lead to significant casting quality problems. It is difficult to xe2x80x9cwarmxe2x80x9d the type of chill described above, as a result of the inherent time delay from mold assembly to mold filling.
Another method to rapidly cool portions of the casting involves using the semi-permanent molding (SPM) process. This method employs convective cooling of permanent mold tooling by water, air or other fluid. In the SPM process, the mold package is placed into the SPM machine. The SPM machine includes an actively cooled permanent (reusable) tool designed to shape some portion of the bulkhead features. The mold is filled with metal. After several minutes have passed, the mold package and casting are separated from the permanent mold tool and the casting cycle is repeated. Such machines typically employ multiple molding stations to make efficient use of the melting and mold filling equipment. This leads to undesirable system complexity and difficulty in achieving process repeatability.
In past manufacture of an aluminum engine V-block with cast-in-place bore liners using separate crankcase cores and barrel cores with liners thereon, the block must be machined in a manner to insure, among other things, that the cylinder bores (formed from the bore liners positioned on the barrel features of the barrel cores) have uniform bore liner wall thickness, and other critical block features are accurately machined. This requires the liners to be accurately positioned relative to one another within the casting, and that the block is optimally positioned relative to the machining equipment.
The position of the bore liners relative to one another within a casting is determined in large part by the dimensional accuracy and assembly clearances of the mold components (cores) used to support the bore liners during the filling of the mold. The use of multiple mold components to support the liners leads to variation in the position of the liners, due to the accumulation, or xe2x80x9cstack-upxe2x80x9d of dimensional variation and assembly clearances of the multiple mold components.
To prepare the cast V-block for machining, it is held in either a so-called OP10 or a xe2x80x9cqualificationxe2x80x9d fixture while a milling machine accurately prepares flat, smooth reference sites (machine line locator surfaces) on the cast V-block that are later used to position the V-block in other machining fixtures at the engine block machining plant. The OP10 fixture is typically present at the engine block machining plant, while the xe2x80x9cqualificationxe2x80x9d fixture is typically present at the foundry producing the cast blocks. The purpose of either fixture is to provide qualified locator surfaces on the cast engine block. The features on the casting which position the casting in the OP10 or qualification fixture are known as xe2x80x9ccasting locatorsxe2x80x9d. Typically, the OP10 or qualification fixture for V-blocks with cast-in-place bore liners uses as casting locators the curved inside surface of at least one cylinder bore liner from each bank of cylinders. Using curved surfaces as casting locators is disadvantageous because moving the casting in a single direction causes a complex change in spatial orientation of the casting. This is further compounded by using at least one liner surface from each bank, as the banks are aligned at an angle to one another. As a practical matter, machinists prefer to design fixtures that first receive and support a casting on three xe2x80x9cprimaryxe2x80x9d casting locators that a establish a reference plane. The casting then is moved against two xe2x80x9csecondaryxe2x80x9d casting locators, establishing a reference line. Finally, the casting is moved along that line until a single xe2x80x9ctertiaryxe2x80x9d casting locator establishes a reference point. The orientation of the casting is now fully established. The casting is then clamped in place while machining is performed. The use of curved and angled surfaces to orient the casting in the OP10 or xe2x80x9cqualificationxe2x80x9d fixture can result in less precise positioning in the fixture and ultimately in less precise machining of the cast V-block, because the result of moving the casting in a given direction, prior to clamping in position for machining, is complex and potentially non-repeatable.
An object of the invention is to use an integral barrel crankcase core in the production of aluminum and other engine V-blocks that include cast-in-place bore liners where the barrel features are adapted to receive cylinder bore liners in a manner that the liners and casting locators are accurately positioned one to the other in the mold package and in the cast engine block produced in the mold package.
Another object of the present invention is to provide method and apparatus for sand casting of engine cylinder blocks in a manner that overcomes one or more of the above disadvantages.
The present invention involves method and apparatus for assembling an engine block mold package as well as a mold package and an integral barrel crankcase core. In an embodiment of the invention, the integral barrel crankcase core includes a plurality of barrels in two banks on an integral crankcase region. The barrels are formed by respective barrel-forming tool elements of a core box. The barrel-forming tool elements are configured to also form one or more casting locator surfaces on the crankcase region. Because the casting locator surface(s) is/are formed on the crankcase region using the same tool elements that also form the barrels, the casting locator surface(s) are consistently and accurately positioned relative to the barrels and thus the cylinders to be formed in the engine block casting. The locator surface(s) can be used to locate the engine block casting in subsequent aligning and machining operations without the need to reference an inside curved surface of cylinder bore liners.
Pursuant to an illustrative embodiment of the invention, an integral barrel-crankcase core is formed in core box tooling having two movable barrel-forming tool elements that also form the casting locator surfaces on the crankcase region when the barrels themselves are formed. The barrel-forming tools elements are configured to form primary, secondary, and tertiary casting locator surfaces on the crankcase region of the core.
Advantages and objects of the present invention will be better understood from the following detailed description of the invention taken with the following drawings.